In a plant for producing acetic acid using a methanol method, carbon monoxide is allowed to react with methanol in a carbonylation reactor, the reaction solution is vaporized by an evaporator (flasher), and the resulting vaporized product is further distilled by a distillation column for separation to produce acetic acid. Since such steps generates reaction heat due to the carbonylation reaction, it is necessary to remove the reaction heat. As a system for removing the reaction heat in this plant, a method of removing the reaction heat by utilizing a latent heat of evaporation of acetic acid generated in an evaporator and a latent heat of evaporation of constituents (e.g., methyl iodide, methyl acetate, water, and other by-products) of the reaction solution accompanying acetic acid evaporation is effective. In particular, this heat removal method is used according to the plant size and is effective in a conventional plant in which the amount of production per plant is about 200,000 t/year (a heating value of about 1.36×107 kcal/hour), and the heat removal method is economical in the respect that the flash amount necessary for separating and collecting acetic acid produced in the reactor is well-proportioned by excellent material balance and heat balance. In these years, however, the economical scale of the acetic acid production plant is increasing to an amount of production of not less than about 400,000 t/year per plant, one reason of which is the growth in demand. For the scale of an amount of production of not less than 250,000 t/year, it is necessary to remove the quantity of heat larger than the latent heat of evaporation of the produced acetic acid or the like. Thus, only the heat removal method utilizing the latent heat of evaporation is insufficient for removing the reaction heat.
Regarding a system for removing a reaction heat, U.S. Pat. No. 5,374,774 publication (Patent Document 1) discloses a method for controlling a liquid surface of a reactor and that of an evaporator in a process for producing acetic acid using a methanol method. In this document, as a method for controlling the temperature of the reactor, a method for providing a recycle line, for recycling a catalyst from a bottom solution of the evaporator to the reactor, with a cooling unit, and a method for providing the reactor with a cooler (a cooling unit) are described.
However, according to the method for providing the catalyst recycle line with the cooling unit, since sensible heat is used for the heat removal, it is necessary to install an expensive equipment using a high-grade material with a high corrosion resistance, and the amount of removed heat is small. Moreover, according to the method for providing the reactor with the cooling unit, since a reaction solution as well as the reactor is exposed to a high temperature and a high pressure, carbon monoxide dissolved in the reaction solution is consumed in pipelines. Thus, it is difficult to maintain an active state of a rhodium-carbonyl complex as an active catalyst species (for example, [RhI2(CO)2]−, [RhI2(CO)4]−). The main catalyst Rh does not contribute to the reaction due to sedimentation thereof, e.g., in the state of RhI3, or the sediment blockades the pipeline or the like and causes incomplete actions of a valve, a pump, or the like, and it is difficult to operate the system stably. Further, the method requires use of a high-grade material with a high corrosion resistance. However, the equipment for the use under a high temperature and a high pressure is expensive, and the reaction solution is easy to leak. Thus, it is difficult to operate the system stably.
Moreover, Japanese Patent No. 3616400 publication (JP-3616400B, Patent Document 2) discloses a process for purifying an acetic acid product using a single distillation column, which comprises operating a liquid-phase carbonylation reaction by controlling a formulation thereof. This document discloses that the loss due to evaporation during the process is minimalized by cooling a vapor from a flash tank and a non-condensation product from the head of the distillation column. Specifically, in a series of steps for recycling the cooled condensate to a reaction vessel, most of the quantity of heat of the reaction solution withdrawn from the reaction vessel is transferred as the quantity of heat of a flash vapor and removed by condensing the flash vapor in a condenser installed to the top of the distillation column.
However, in the method for removing heat by the condenser installed to the top of the distillation column, the condenser is subjected to a large load, and the cost of equipment or the energy cost is so large that it can no more be disregarded. In particular, for the recent large-sized plant, the size of the condenser and that of the distillation column have to be increased.
Further, Japanese Patent Application Laid-Open No. 2002-255890 (JP-2002-255890A, Patent Document 3) discloses a method for controlling a temperature of a reaction system by controlling a quantity of heat according to a circulation amount of a separated component which is separated in a step for separating a reaction product. In the method, the quantity of heat is controlled by installing a temperature control unit (a heat exchanger) and a control unit in a circulation line for returning a higher boiling point component separated in a flash distillation column to the reaction system.
This method is effective in stably operating the system. However, for a large-sized plant, only cooling of the liquid (sensible heat) is insufficient to remove the heat.